A former U.S. Marine Corps colonel and combat veteran of 24 years resigned in late September, now saying he did so because of President Donald Trump and “concern for our country’s future.”

In an op-ed titled “I resigned from the military because of Trump,” published in The Washington Post on Thursday, Doug Krugman noted that “no commander in chief is perfect,” but said he nevertheless believed that previous presidents took their oaths to the Constitution seriously.

With Trump, he wrote, he no longer believes that.